Microwave drying system using phase shifters



Aug. 26, 1969 w. J. BLEACKLEY MICROWAVE DRYING SYSTEM USING PHASESHIFTERS Filed May 51. 1967 WVENTOR w/u/AH J. BLFACA/ZF) 6y Y A'flvUnited States Patent O" 3,463,894 MICROWAVE DRYING SYSTEM USING PHASESHIFTERS William J. Bleackley, Ottawa, Ontario, Canada, assignor toCanadian Patents and Development Limited, Ottawa, Ontario, Canada, acorporation of Canada Filed May 31, 1967, Ser. No. 642,575 Claimspriority, application Canada, July 8, 1966 964,965 Int. Cl. H05b 9/06US. Cl. 21910.61 4 Claims 'ABSTRACT OF THE DISCLOSURE A microwave dryingsystem for film, paper, and the like of the type using a tuner in theinput to provide a tuned circuit in the drying section wherein theundesirable effects of stationary standing waves on the material beingdried are eliminated by cyclically shifting the standing wave pattern inthe drying section by means of phase shifters placed ahead of and afterthe drying section.

This invention relates to a microwave drying system wherein theundesirable effects due to standing waves in the system are eliminatedor greatly reduced.

Microwave energy is coming into widespread use for the rapid andeflicient drying of many materials. It has been especially applied tothe drying of lumber, paper, film, leather and many other materials thatrequire moisture removed from them without particular damage to thematerial itself. In general the systems that have been developed for thedrying of flat or web-like material such as paper and film involve asuitably electrically energized section of wave-guide with relativelynarrow slots in the broad faces. The material to be dried passes throughthe interior of the wave-guide via the slots where the electric fielddifferentially heats the water which evaporates out of the material andis removed by a stream of air. The water is highly lossy over much ofthe microwave frequency spectrum whereas the paper or film material isnormally quite unaffected provided the system is such that there are nogreat concentrations of energy at certain points in the system. Anefiicient and rapid drying system may be readily designad using theseconcepts but great care must be exercised in the actual design of aworkable unit.

In some drying systems, i.e., the travelling wave type, the energy afterpassing through the drying portions of the wave-guide is directed into asuitable termination or load that absorbs the excess energy. Ideally nostanding waves are set up and there is no problem of hot spots andburning or injury to the material being dried. However, in the dryingsystem using a short-circuit termination and a tuner at the input toprovide a tuned or resonant circuit, standing waves are set up and theseinvolve concentrations of microwave energy at localized positions acrossthe material being dried. This can result in hot spots and the burningor injury to the material at these positions. The tuned or resonantsystem is most attractive from the economic viewpoint because all of theenergy is used in the drying process with none being absorbed and lostin an energy absorbing load.

It is, therefore, an object of the present invention to provide amicrowave drying system in which the effect of standing waves iseliminated or greatly reduced.

It is another object of the invention to provide a microwave dryingsystem of the tuned type in which the undesirable effect of stationarystanding waves is eliminated or greatly reduced but in which there areno adverse or upsetting effects on the tuning system and the microwavepower supply.

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It is another object of the invention to provide a microwave dryingsystem for paper, film, and the like that provides even drying actionacross the width of the material.

It is another object of the invention to provide a microwave dryingsystem that is relatively easy to operate and that requires no complex,mechanical apparatus.

These and other objects of the invention are achieved byproviding amicrowave drying system for paper, film, and the like wherein themicrowave energy before and after it passes through the drying portionsof the system passes through phase shifters such that the standing wavesin the system are reciprocated so that any undesirable ef fects that thestanding waves might otherwise have on the material to be dried isaveraged out, said phase shifters being interconnected such that noundersirable effect is placed on the energizing system.

In drawings which illustrate embodiments of the invention, the figure isan isometric view of a microwave drying system including phase-shifters.

Referring to the figure a microwave drying system is shown. A wave-guide10 has a series of slots 11 through which passes a film 12 unwindingfrom reel 13 onto reel 14, the latter being driven by a gear reductionmotor 15. As the film passes through the wave-guide, the microwaveenergy is absorbed by the water in the film and boils off as steam. Thefilm itself is normally unaflected provided it is moved through thewave-guide at a reasonable speed and the wave-guide itself is terminatedin a load or termination that absorbs any energy left after the dryingprocess. If, however, there are standing waves existing in thewave-guide and this is normally always the case although usually muchmore definitely where the wave-guide is terminated in a short-circuitwith the energy being reflected back along the guide, there is atendency for hot spots to form due to the concentration of the electricfield at the standing wave nodes. This results in burning or injury tothe film or paper and is most undesirable.

The system is energized by a suitable power source 16, e.g. magnetron.An isolator 18 inserted in the line by means of flanges 18a and 18b isused to protect the magnetron. A microwave tuner 19 is used to tune thewave-guide system to make it in effect a resonant circuit and to prevent reflections from reaching the power source. Although any good tunermight be used for this application, a type similar to that shown inUnited States Patent No. 3,110,- 002, dated Nov. 5, 1963, and entitledVariable Insertion Sliding Post-Slotted Line Tuner Having MeansPreventing Energy Loss Past Sides of Post, would operate most suitably.A direction coupler 20 which is inserted in the line by means of flanges20a and 20b connects to a detector 22and suitable load 21. The output ofthe detector is read on meter 23 to give the operator timinginformation.

A first phase shifter 24 is located in the system ahead of thewave-guide 10 and comprises a helical assembly 26 mounted on shaft 28.The helical assembly comprising a worm core of dielectric materialrotates inside a coaxial cable 25 which is in the form of a cylindricalhelix of pre-determined pitch mounted inside backing material 27 with ahelical groove 27a left therebetween for passage of a coolant ifnecessary. Because the phase shifter involves a co-axial cable deviceprovision is made for transition from wave-guide operation to co-axialcable inside housing 24a and in the reverse sense in housing 24b.Provision for the introduction of cooling water is made by inlet andoutlet ducts 30 and 29. The phase shifter is connected to the wave-guide10 by suitable flange 31. An identical phase shifter 33 is connected onthe output end of the wave-guide system by flange 32 and incorporatesworm core 35 mounted on shaft 37, helical co-axial cable 34, transitionhousings 33a and 33b, and coolant inlet and outlet ducts 38 and 39. Bothphaseshifters are driven from common-shaft 41 by suitable motor drivemeans 42. The type of phase-shifter shown here is described in much moredetail in United States Patent No. 3,145,353, dated Aug. 18, 1964 andentitled Variable Delay Using Dielectric Screw Rotatable InsideSurrounding Helical Transmission Line, and reference should be made tothat patent for more information on the theory of operation andconstruction of the device.

The complete system is terminated at 40 by a suitable short circuittermination. It will be realized that the system described above is mostsuitable for the drying of film where it is difficult or inconvenient touse all'the energy in a one way operation, i.e., where the energy islargely used in the drying operation and any remaining power is absorbedby a suitable load or termination. It is desirable especially in thecase of film to reflect the energy so that it will substantially all beused in the drying process. This latter mode of operation, however,involves the formation of standing wave patterns and the attendantdifficulties described above.

In operation the two phase-shifters are driven in synchronism but 180out of phase such that the phase is being shifted (advanced) at one andretarded at the other. The speed of rotation (phase shift) is chosen inrelation to the speed of travel of the film or paper 12 and would be inthe order of 1200 to 1800 rpm. for a typical case. The complementaryaction of the phase shifters is such that the standing wave pattern inwave-guide oscillates or cyclically moves along its length (across thewidth of the film 10) and any tendency for burning or injury to the filmor material is averaged or cancelled out.

Other forms of phase-shifters may be used but they must be able tohandle the relatively large amounts of power involved in these systemsand they must be able to operate sufiiciently quickly in relation offilm transport speed to be effective. Mechanical devices such as linestretchers, trombone sections, etc. might be used but the aboveconsiderations largely dictate against their ready use. Electrical phaseshifters are, therefore, much more convenient but must be able tooperate in complementary fashion as described above. It will be realizedthat although the phase shifters alter the standing wave patterns insidethe drying portions of the wave-guide system no effect is seen by thepower source or the termination.

What is claimed is: 1. A microwave drying system for film, paper, andthe like comprising:

(a) a wave-guide section having slotsin its broad faces for the passageof the material to be dried, and which when energized generates astanding Wave pattern,

(b) a microwave power source connected to said wave-guide section forelectrically energizing it such that moisture in the material to bedried will differentially absorb the microwave energy and evaporate off,

(c) a short circuit termination terminating said waveguide section,

(d) a first phase shifter inserted in the system between said powersource and said wave-guide section and,

(e) a second phase shifter inserted in the system between saidwave-guide section and said termination,

(f) said first and second phase shifters being operated in synchronousout-of-phase relation such that the phase at one is being advanced whilethe phase at the other is being retarded.

2. A microwave drying system as in claim 1 wherein the means forshifting the standing wave pattern are electrical phase-shifters.

3. A microwave drying system as in claim 1 wherein the phase shifterscomprise a helical worm of high loss tangent dielectric materialrotating inside a helical coaxial cable, the said cable having a recesscut on its inner surface such as to electrically expose the innerconductor of the co-axial cable to the said worm.

4. In a microwave drying system of the type wherein the material to bedried is passed, through an electrically energized short circuitterminated wave-guide such that microwave energy is absorbed by themoisture in the material, the improvement comprising means forcyclically shifting the phase of the standing wave pattern in thewave-guide said means comprising a phase shifter inserted in the systemahead of the drying wave-guide and a phase shifter inserted in thesystem after the drying wave-guide, said phase shifters being operatedsuch that the phase is being advanced at one While being retarded at theother.

References Cited UNITED STATES PATENTS 2,640,142 5/1953 Kinn 21910.61 X

3,145,353 8/1964 Bleackley 333-24.2

FOREIGN PATENTS 1,048,317 11/1966 Great Britain.

1,370,675 7/ 1964 France.

1,132,268 6/1962 Germany.

JOSEPH V. TRUHE, Primary Examiner L. H. BENDER, Assistant Examiner U.S.Cl. X.R. 21910.55

